WARNING!! This Review may contain Spoilers!!
Think of an X-Men story, any single one of them. Days Of Future Past, God Loves Man Kills, Mutant Genesis, Age Of Apocalypse, E For Extinction, Unstoppable, Messiah Complex, Second Coming etc, these are great comics. But The Dark Phoenix Saga is better by a long way. Compared to this, only the likes of comic book royalty such as The Dark Knight Returns, Maus and Scott Pilgrim's Finest Hour are close.
Written by Chris Claremont, X-Men writer for a massive 16 years between 1975-1991, and co-plotted by artist John Byrne, The Dark Phoenix Saga effectively rounds off a story that began in 1976 with the transformation/possession of original X-Man Jean Grey (then going by the fairly crap name of Marvel Girl) into/by Phoenix, the most powerful being in creation. As the title suggests, the book chronicles the descent of Phoenix into darkness. Containing issues #129-1937, it also contains the debuts of two well known X-Men: Kitty Pyde (a.k.a: Sprite/Ariel/Shadowcat) and Alison Blaire (Dazzler).
You have noticed that I haven't said much about the actual story. This is because, for me, The Dark Phoenix Saga is special, and I don't want to spoil it. The art is Superb, not just for the period, but now as well. Not quite as good as in his run on Fantastic Four, but still absolutely indispensable. Claremont's writing is at it's peak by this point, and besides being more wordy then some are used to now-a-days, he nails every characters voices and thoughts with dynamite precision.
Basically, it's bloody spectacular. And to me, it's never been bested. Yet.